Hello there, @JMM2021.

The best way to investigate the two checks that your supplier didn't receive is by contacting the Melio Customer Support Team. One of their representatives will guide you in tracking the payment and do a further investigation with the two Melio checks.

You can go to this link and go to the Contact page below App information to get their phone number.

You can also run the Bills and Applied Payments report to review the payments you’ve made (via Melio) and the bills that go with them. Then, you can customize it to show the tracking numbers and other details.

In case you need some related article in managing your vendors and transactions, you can always visit our Income and Expenses article for reference.

If you have other questions for QuickBooks Online, please let me know. I'm here to help.